1. Field of the Invention
The invention relates to a nickel/aluminum oxide catalyst having a nickel content from 10 to 60% by weight, based on the total catalyst, a process for the preparation thereof and also the use thereof for hydrogenating aromatic hydrocarbons.
In addition, the invention relates to a process for hydrogenating aromatic hydrocarbons with the aid of the catalyst.
2. Discussion of the Background
Mineral oil fractions are used in many areas, such as in the surface coating industry, and as solvents for surface coatings.
Reduction of the aromatics content of the mineral oil fractions allows the isolation of de-aromatized products which are used, for example, in the food industry, pharmaceuticals and even in the cosmetic industry.
Furthermore, aromatics-reduced kerosene can also be used as jet fuel. The reduction in the aromatics content gives advantages, for example in the form of an improved smoke point of the jet fuel.
The lowering of the aromatics content can in general be achieved in a number of ways, as are described, for example, in DE-A 23 05 143. For instance, aromatics-containing hydrocarbons can be de-aromatized by treatment with sulfuric acid or oleum. This technique is, however, problematical because of material attrition by corrosion. Furthermore, handling sulfuric acid or oleum is problematical from the point of view of disposal of the waste sulfuric acid. Another method of reducing the aromatics content is the hydrogenation of the aromatic hydrocarbons with the aid of suitable catalysts. For instance, DE-A 20 42 166 describes catalyst systems for hydrogenating aromatic hydrocarbons using Pt or Rh on catalyst supports. However, such nobel metal catalysts are relatively expensive because of their noble metal content and thus adversely affect the economics of the hydrogenation process.
To make the application of the mineral oil fractions possible, for example in the food industry, the aromatic content of the mineral oil fractions has to be reduced considerably. If the reduction of the aromatics content is to be achieved by catalytic hydrogenation, then lowering the aromatics content to a very low level, for example, in the concentration range of a few ppm by weight, requires large catalyst volumes, which substantially affects the economics of the catalytic hydrogenation process.
DE-A 23 05 143 discloses a catalyst system based on nickel and Al.sub.2 O.sub.3. The preparation of the catalyst is carried out via the precipitation of nickel/aluminum hydroxycarbonate with various further processing steps. The preparation of the catalyst includes, inter alia, the compounding of the precipitated nickel/aluminum complex with Al.sub.2 O.sub.3. This process is comparatively complicated. The catalyst thus obtained can be used for hydrogenating aromatic hydrocarbons. However, a residual aromatics content of about 200 ppm by weight still exists in the hydrogenation product after hydrogenation. In addition, the space velocity over the catalyst, expressed in [g of throughput/(cm.sup.3 of catalyst.h)], is, at about 1 g/(cm.sup.3.h), not very high.
EP-A 0 290 100 describes a catalyst which is prepared by impregnation of a preformed Al.sub.2 O.sub.3 support with a nickel salt solution. Nickel crystallite sizes of about 2 to 8 nm can be calculated from the features claimed. It is conventionally assumed that nickel catalysts are more active the smaller the crystallite diameter. The catalyst support is pre-treated at high temperatures prior to impregnation, so as to achieve a particular modification of the Al.sub.2 O.sub.3. The impregnation technique makes it possible to achieve only comparatively low nickel loadings on the catalyst support. In order to increase the nickel content of the catalyst to values of industrial interest, it is necessary to carry out the impregnation of the catalyst a number of times. After each impregnation stage, the catalyst has to be dried or calcined. This process makes the preparation of the catalyst complicated, particularly when high nickel concentrations in the catalyst are to be achieved. When using this catalyst according to EP-A 0 290 100, conversions of 96% in the hydrogenation of aromatic hydrocarbons are only achieved at temperatures of about 150.degree. C. The residual aromatics content in the hydrogenation product is about 7600 ppm by weight under the above-described conditions. At temperatures of about 175.degree. C., the conversion is about 99.4% and the residual aromatics concentration is about 1300 ppm by weights.
EP-A 0 092 878 teaches catalysts containing nickel on Al.sub.2 O.sub.3, which are likewise used for hydrogenating aromatic hydrocarbons. Nickel crystallite sizes of about 2 to 8 nm are claimed. The preparation of these catalysts is carried out via precipitation of nickel hydroxide on a transition aluminum oxide. This process of preparation is comparatively complicated because of the various sub-steps, particularly when high nickel contents on the catalyst are required, which is the case, for example, if contamination of the catalyst by catalyst poisons cannot be ruled out. A need therefore continues to exist for a nickel catalyst of improved aromatic hydrogenation ability.